Pololu A-Star 32U4 Prime LV, A-Star 32U4 Prime LV microSD (soldered only with SMD components), A-Star 32U4 Prime LV microSD with LCD display, A-Star 32U4 Prime LV (soldered only with SMD components), A-Star 32U4 Prime LV microSD User's manual
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
Pololu A-Star 32U4 User’s
Guide
https://www.pololu.com/docs/0J61/all Page 1 of 62
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Supported operating systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2. Contacting Pololu . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3. A-Star 32U4 Micro . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.1. A-Star 32U4 Micro pinout and components . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.2. A-Star 32U4 Micro schematic and dimensions . . . . . . . . . . . . . . . . . . . . . . 10
4. A-Star 32U4 Mini . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.1. A-Star 32U4 Mini pinout and components . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.2. A-Star 32U4 Mini ULV regulator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
4.3. A-Star 32U4 Mini LV regulator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
4.4. A-Star 32U4 Mini SV regulators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
4.5. A-Star 32U4 Mini schematic and dimensions . . . . . . . . . . . . . . . . . . . . . . . 22
5. A-Star 32U4 Prime . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
5.1. A-Star 32U4 Prime configurations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
5.2. A-Star 32U4 Prime pinout and components . . . . . . . . . . . . . . . . . . . . . . . . 25
5.3. A-Star 32U4 Prime LV regulator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
5.4. A-Star 32U4 Prime SV regulator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
5.5. A-Star 32U4 Prime schematic and dimensions . . . . . . . . . . . . . . . . . . . . . . 38
5.6. A-Star 32U4 Prime demo program . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
6. Getting started . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
6.1. Installing Windows drivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
6.2. Programming using the Arduino IDE . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
6.3. Programming using avr-gcc and AVRDUDE . . . . . . . . . . . . . . . . . . . . . . . 49
7. A-Star 32U4 Arduino library . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
8. The A-Star 32U4 USB interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
9. The A-Star 32U4 Bootloader . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
10. Reviving an unresponsive A-Star . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
10.1. Reviving using the Arduino IDE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
10.2. Reviving using AVRDUDE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
11. Related Resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
Page 2 of 62
• Programmable 16 MHz Atmel
ATmega32U4 AVR microcontroller
◦ 32 KB flash (4 KB used by
bootloader, leaving 28 KB
available for user program by
default)
◦ 2.5 KB SRAM
◦ 1 KB EEPROM
◦ Native full-speed USB
(12 Mbps)
• Preloaded with Arduino-compatible
bootloader
• Can be powered from USB or external
source regulated to 5 V by onboard
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
1. Overview
The Pololu A-Star 32U4 microcontroller boards
are general-purpose programmable modules
based on Atmel’s ATmega32U4 AVR
microcontroller, which has 32 KB of flash
program memory, 2.5 KB of RAM, and built-in
USB functionality. Each A-Star (abbreviated A*)
adds onboard components and connectors that
support the microcontroller and make it easier to
use. The boards feature USB interfaces and ship
with a preloaded Arduino-compatible bootloader,
and we provide a software add-on that enables
them to be easily programmed from the Arduino
environment. The following sections of this
user’s guide discuss each A-Star in more detail.
A USB A to Micro-B cable
[https://www.pololu.com/product/2072] (not
included) is required to connect an A-
Star 32U4 to a computer.
Features
From top to bottom: A-Star 32U4 Micro, Mini SV,
and Prime SV.
1. Overview Page 3 of 62
regulator
• Reverse-voltage protection on external power input
• 6-pin ISP header for use with an external programmer [https://www.pololu.com/product/3172]
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
1. Overview Page 4 of 62
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
A-Star comparison table
A-Star 32U4 Mini
ULV
A-Star 328PB
Micro
Microcontroller: ATmega328PB ATmega32U4
User I/O lines: 24 18 26
PWM outputs: 9 7 7 7
Analog inputs: 8 8 12 12
Ground access
points:
User LEDs: 1 2 3 3 3
User
pushbuttons:
USB interface:
Reset button:
Power switch:
Buzzer option:
microSD option:
LCD option:
Motor drivers:
Operating
voltage:
Regulator type: 3.3 V or 5 V linear 5 V linear
Regulated
current:
Dimensions: 1.3″ × 0.7″ 1″ × 0.6″ 1.9″ × 0.7″ 2.8″ × 2.1″ 2.6″ × 2.2″
Weight:
1 Some microcontroller resources are used by on-board hardware.
2 These values are rough approximations for comparison purposes. Available current depends on input voltage, current consumed
by the board, ambient conditions, and regulator topology. See product documentation and performance graphs for details.
3 Without included optional headers.
(2)
3.3V VCC: 3.8 V to
6 2 4 43 44
— — — 3 3
15 V
5V VCC: 5.5 V to
15 V
100 mA 100 mA
(3)
1.5 g
A-Star 32U4
Micro
5.5 V to 15 V
(3)
1.3 g
A-Star 32U4 Mini
LV
A-Star 32U4 Mini
SV
ULV: 0.5 V to
5.5 V
LV: 2.7 V to 11.8 V
SV: 5 V to 40 V
5 V switching
ULV: step-up
LV: step-up/step-
down
SV: step-down
ULV: 500 mA
LV: 1 A
SV: 800 mA
(3)
3.4 g
A-Star 32U4
Prime LV
A-Star 32U4
Prime SV
(1)
26
LV: 2 V to 16 V
SV: 5 V to 36 V
5 V switching
LV: step-up/step-
down
SV: step-down
LV: 1.8 A
SV: 1 A
13 g to 33 g 14 g to 23 g
A-Star 32U4 Robot
Controller LV
A-Star 32U4 Robot
Controller SV
26
7
12
LV: 2.7 V to 11 V
SV: 5.5 V to 36 V
5 V switching
LV: step-up/step-down
SV: step-down
LV: 1 A
SV: 1.5 A
(1)
(1)
(1)
1. Overview Page 5 of 62
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
1.1. Supported operating systems
The A-Star 32U4 boards can be programmed using any operating system that supports the Arduino
environment. We have tested the A-Stars, our Arduino software add-on, and the Arduino IDE on
Microsoft Windows 10, 8.1, 8, 7, Vista, XP (with Service Pack 3), Linux, and Mac OS X.
1. Overview Page 6 of 62
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
2. Contacting Pololu
We would be delighted to hear from you about any of your
projects and about your experience with the Pololu A-Stars. You
can contact us [https://www.pololu.com/contact] directly or post on
our forum [http://forum.pololu.com/]. Tell us what we did well, what
we could improve, what you would like to see in the future, or
anything else you would like to say!
2. Contacting Pololu Page 7 of 62
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
3. A-Star 32U4 Micro
A-Star 32U4 Micro,
top view.
3.1. A-Star 32U4 Micro pinout and components
Pinout
The diagram above identifies the I/O and power pins on the A-Star 32U4 Micro; it is also available
as a printable PDF [https://www.pololu.com/file/0J796/a-star-32u4-micro-pinout.pdf] (409k pdf). For more
information about the ATmega32U4 microcontroller on this board, see Atmel’s ATmega32U4
3. A-Star 32U4 Micro Page 8 of 62
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
documentation [https://www.microchip.com/wwwproducts/en/ATmega32u4].
Printed on the A* circuit board are indicators that you can use to quickly identify each pin’s capabilities:
a triangle next to the pin means it can be used as an analog input, and a square wave symbol under
the pin number means it can be used as a PWM output.
LEDs
The A-Star 32U4 Micro has two indicator LEDs.
The yellow LED is connected to Arduino pin 13, or PC7. You can drive this pin high in a user program
to turn this LED on. The A-Star 32U4 Bootloader [https://www.pololu.com/docs/0J61/9] fades this LED on
and off while it is waiting for a sketch to be loaded.
The green LED is connected to PD5 and lights when the pin is driven low. While the board is running
the A-Star 32U4 Bootloader or a program compiled in the Arduino environment, it will flash this LED
when it is transmitting data via the USB connection.
Connectors
The A-Star 32U4 includes a USB Micro-B connector that can be used to connect to a computer’s
USB port via a USB A to Micro-B cable [https://www.pololu.com/product/2072] (not included). The USB
connection can be used to transmit and receive data from the computer, and a preloaded USB
bootloader makes it possible to program the board over USB. The USB connection can also provide
power to the A-Star.
The board also has a 6-pin ISP header that allows it to be programmed with an external programmer,
such as our USB AVR programmer v2.1 [https://www.pololu.com/product/3172]. Pin 1 of the header is
indicated with a small white dot and has an octagonal shape. Three of the pins on this header can
be used as an SPI interface or as general-purpose digital I/O, as shown in the pinout diagram. In
the Arduino environment, you can refer to these three pins using either their pin numbers or the
names of their SPI functions (which are defined as aliases); for example, digitalRead(15) and
digitalRead(SCK) are equivalent.
Power
The A-Star 32U4 Micro can either be powered directly from the USB 5 V supply or from a separate
source on the VIN pin. The board features a power selection circuit that allows both USB and VIN to
be connected at the same time; if this is done, the A-Star will draw power from VIN.
USB power input: The A-Star can be powered from the USB 5 V bus voltage (VBUS) if it is connected
to a USB cable. It will draw power from USB only if VIN is disconnected. A resettable PTC fuse on
VBUS makes it less likely for the A-Star (and the connected computer or other device) to be damaged
3. A-Star 32U4 Micro Page 9 of 62
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
if too much current is drawn from the USB connection.
VIN power input: The A-Star can be powered from VIN if you connect a 5.5 V to 15 V power
supply (such as a battery or wall power adapter) to the VIN and GND pins, with the positive terminal
connected to VIN.
When powering the A-Star 32U4 Micro from VIN, a minimum voltage of 5.5 V is required
to ensure that the board’s 5 V supply is stable. Even if power is being provided to the
A-Star via USB, connecting a voltage higher than 0 V but lower than 5.5 V to VIN is not
recommended, as this can interfere with the power selection circuit and cause the 5 V line
to drop (potentially triggering a brown-out reset).
5V power output: This pin provides access to the board’s 5 V supply, which comes from either the
USB 5 V bus voltage or a low-dropout (LDO) regulator on VIN, depending on which power source is
connected. The regulator can supply up to 100 mA, although some of this is used by the board itself
(typically about 25 mA) or used to provide current for the GPIO pins or 3.3 V power output (see below).
3V3 power output: This pin gives access to the output of the internal 3.3 V regulator inside the
ATmega32U4. The microcontroller uses this regulated voltage for USB signaling, but up to about
50 mA is available for powering external circuits or devices.
When the A-Star 32U4 Micro is being powered through VIN, the sum of the 5V output current, 3V3
output current, GPIO output current, and current used by the board itself should not exceed the 100
mA that the regulator can provide.
3.2. A-Star 32U4 Micro schematic and dimensions
Schematic diagram
3. A-Star 32U4 Micro Page 10 of 62
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
Pololu A-Star 32U4 Micro schematic diagram.
This schematic is also available as a PDF: A-Star 32U4 Micro schematic diagram
[https://www.pololu.com/file/0J742/pololu-a-star-32u4-micro-schematic-diagram.pdf] (253k pdf).
3. A-Star 32U4 Micro Page 11 of 62
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
Dimension diagram
A dimension diagram of the A-Star 32U4 Micro is available as a PDF: A-Star 32U4 Micro dimension
diagram [https://www.pololu.com/file/0J747/pololu-a-star-32u4-micro-dimension-diagram.pdf] (255k pdf).
3. A-Star 32U4 Micro Page 12 of 62
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
4. A-Star 32U4 Mini
A-Star 32U4 Mini ULV, LV, and SV.
4.1. A-Star 32U4 Mini pinout and components
Pinout
4. A-Star 32U4 Mini Page 13 of 62
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
This diagram identifies the I/O and power pins on the A-Star 32U4 Mini (ULV, LV, and SV versions);
it is also available as a printable PDF [https://www.pololu.com/file/0J1515/a-star-32u4-mini-pinout.pdf] (223k
pdf). For more information about the ATmega32U4 microcontroller and its peripherals, see Atmel’s
ATmega32U4 documentation.
Printed on the A* circuit board are indicators that you can use to quickly identify each pin’s capabilities:
a triangle next to the pin means it can be used as an analog input, and a square wave symbol under
the pin number means it can be used as a PWM output.
LEDs
The A-Star 32U4 Mini has three indicator LEDs.
The yellow LED is connected to Arduino pin 13, or PC7. You can drive this pin high in a user program
to turn this LED on. The A-Star 32U4 Bootloader [https://www.pololu.com/docs/0J61/9] fades this LED on
and off while it is waiting for a sketch to be loaded.
The green LED is connected to the pin labeled TXL, or PD5, and lights when the pin is driven
low. While the board is running the A-Star 32U4 Bootloader or a program compiled in the Arduino
environment, it will flash this LED when it is transmitting data via the USB connection.
4. A-Star 32U4 Mini Page 14 of 62
• ULV: 0.5 V to 5.5 V (see Section 4.2 for regulator details)
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
The red LED is connected to the pin labeled RXL (usable as Arduino pin 17), or PB0, and lights when
the pin is driven low. While the board is running the A-Star 32U4 Bootloader or a program compiled in
the Arduino environment, it will flash this LED when it is receiving data via the USB connection.
Connectors
The A-Star 32U4 includes a USB Micro-B connector that can be used to connect to a computer’s
USB port via a USB A to Micro-B cable [https://www.pololu.com/product/2072] (not included). The USB
connection can be used to transmit and receive data from the computer, and a preloaded USB
bootloader makes it possible to program the board over USB. The USB connection can also provide
power to the A-Star.
The board also has a 6-pin ISP header that allows it to be programmed with an external programmer,
such as our USB AVR programmer v2.1 [https://www.pololu.com/product/3172]. Pin 1 of the header is
indicated with a small dot and has an octagonal shape.
Power
The A-Star 32U4 Mini can either be powered directly from the USB 5 V supply or from an external
voltage source, which is regulated to 5 V by its onboard switching regulator.
The board’s power selection circuit uses the TPS2113A power multiplexer [https://www.pololu.com/
product/2596] from Texas Instruments to choose whether its 5 V supply is sourced from USB or an
external supply via the regulator, allowing both sources to be connected at the same time and enabling
the A-Star to safely and seamlessly transition between them. The TPS2113A is configured to select
external power unless the regulator output falls below about 4.5 V. If this happens, it will select the
higher of the two sources, which will typically be the USB 5 V bus voltage if the A* is connected to
USB. The currently selected source is indicated by the STAT pin in the middle of the board; this pin
is an open-drain output that is low if the external power source is selected and high-impedance if the
USB supply is selected. The current limit of the TPS2113A is set to about 1.9 A. For more information
about the power multiplexer, see the TPS2113A datasheet [https://www.pololu.com/file/0J771/tps2113a.pdf]
(1MB pdf).
In some situations, it might be undesirable for the A-Star 32U4 Mini to draw power from an external
source when it is connected to USB. If this is the case, the regulator can be disabled by driving the
regulator shutdown pin, SHDN, high; this shuts down the regulator and causes the power mux to fall
back to USB power. For example, this could allow a battery-powered device to turn off the regulator
and avoid draining its battery while it is connected to a computer.
The input voltage range of the regulator depends on the particular version of the A-Star 32U4 Mini:
4. A-Star 32U4 Mini Page 15 of 62
• LV: 2.7 V to 11.8 V (see Section 4.3 for regulator details)
• SV (see Section 4.4 for regulator details):
◦ original ac02c version: 5 V to 36 V
◦ newer ac02f version: 5 V to 40 V
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
Reverse-protected power inputs: The BAT+ and BAT- pins are power inputs with reverse-voltage
protection. These are the recommended pins to use when connecting an external power supply
because they allow the A-Star’s reverse-voltage protection circuit to help prevent it from being
damaged by accidentally-reversed power connections.
VIN power output (or alternative input): When power is supplied through the BAT pins, the VIN
pin can be used as an output to supply reverse-protected power to other devices. Alternatively, the
external supply can be connected directly between VIN and GND, bypassing the reverse-voltage
protection.
5V power output: This pin provides access to the board’s 5 V supply, which comes from either the
USB 5 V bus voltage or the onboard switching regulator, depending on which power sources are
connected and enabled. Note that some of the available current on the 5 V line is used by the board
itself (typically about 30 mA) or used to provide current for the GPIO pins or 3.3 V power output (see
below).
3V3 power output: This pin gives access to the output of the internal 3.3 V regulator inside the
ATmega32U4. The microcontroller uses this regulated voltage for USB signaling, but up to about
50 mA is available for powering external circuits or devices.
When the A-Star 32U4 Mini is being powered through VIN, the sum of the 5V output current, 3V3
output current, GPIO output current, and current used by the board itself should not exceed the
maximum current that the regulator can provide; see the following sections for details about the
regulator on each version.
4.2. A-Star 32U4 Mini ULV regulator
4. A-Star 32U4 Mini Page 16 of 62
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
The A-Star 32U4 Mini ULV can be powered from a
0.5 V to 5.5 V external source. The input voltage is
regulated to 5 V by a TPS61202 switching step-up
(boost) converter from Texas Instruments. (We also
make a standalone regulator [https://www.pololu.com/
product/2562] based on this integrated circuit.)
The regulator’s low minimum input voltage makes it
possible to power this A* with 1 to 3 NiMH, NiCd, or
alkaline cells or from a single lithium cell. Unlike
standard boost regulators, the TPS61202 also features
a linear down-regulation mode that is automatically
enabled when the input voltage exceeds 5 V, allowing
it to handle input voltages as high as 5.5 V.
As shown in the left graph below, the ULV’s switching
A-Star 32U4 Mini ULV, bottom view with
dimensions.
regulator has an efficiency – defined as
(Power out)/(Power in) – of 70% to 90% for most
combinations of input voltage and load.
The A-Star’s components, including the microcontroller and LEDs, draw 30 mA to 40 mA in typical
applications. The rest of the regulator’s achievable output current, which depends on input voltage
as well as ambient conditions, can be used to power other devices. The right graph above shows
output currents at which the voltage regulator’s over-temperature protection typically kicks in after a
few seconds. These currents represent the limit of the regulator’s capability and cannot be sustained
for long periods; a good estimate for the maximum continuous regulator output current is 60% to 70%
of the values shown in the graph.
4.3. A-Star 32U4 Mini LV regulator
4. A-Star 32U4 Mini Page 17 of 62
• A 4-cell battery holder, which might have a
6 V output with fresh alkalines or a 4.0 V
output with partially discharged NiMH cells,
can be used to power this A*.
• A disposable 9 V battery powering the board
can be discharged to under 3 V instead of
cutting out at 6 V, as with typical linear or
step-down regulators.
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
The A-Star 32U4 Mini LV can be powered from a
2.7 V to 11.8 V external source. The input voltage is
regulated to 5 V by a TPS63061 switching step-up/
step-down (buck-boost) converter from Texas
Instruments. (We also make a standalone regulator
[https://www.pololu.com/product/2123] based on this
integrated circuit.)
The regulator’s flexibility in input voltage is especially
well-suited for battery-powered applications in which
the battery voltage begins above 5 V and drops below
5 V as the battery discharges. Without the typical
restriction on the battery voltage staying above 5 V
throughout its life, a wider range of battery types can
be considered. For example:
A-Star 32U4 Mini LV, bottom view with
dimensions.
As shown in the left graph below, the LV’s switching regulator has an efficiency – defined as
(Power out)/(Power in) – of 80% to 90% for most combinations of input voltage and load.
The A-Star’s components, including the microcontroller and LEDs, draw 30 mA to 40 mA in typical
applications. The rest of the regulator’s achievable output current, which depends on input voltage
4. A-Star 32U4 Mini Page 18 of 62
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
as well as ambient conditions, can be used to power other devices. The right graph above shows
output currents at which the voltage regulator’s over-temperature protection typically kicks in after a
few seconds. These currents represent the limit of the regulator’s capability and cannot be sustained
for long periods; a good estimate for the maximum continuous regulator output current is 60% to 70%
of the values shown in the graph.
4.4. A-Star 32U4 Mini SV regulators
There have been two versions of the A-Star 32U4 Mini SV: the original ac02c version
and its replacement, the newer ac02f version. The ac02f version uses an improved
5 V regulator that can deliver more current (800 mA vs 500 mA) and operates to 40 V
instead of 36 V. The easiest way to distinguish between the two versions is via the
silkscreen on the side of the board opposite the USB connector, where the original
version is labeled ac02c and the new version is labeled ac02f. The new version also
uses an ENIG finish, so the plating on the exposed copper looks gold, and it has
unplated mounting holes. See the picture below for a side-by-side comparison:
A-Star 32U4 Mini SV: comparison of
original ac02c version (left) to newer
ac02f version (right).
The A-Star 32U4 Mini SV (ac02c) is the original version of the Mini SV, and it can be powered from
a 5 V to 36 V external source. The input voltage is regulated to 5 V by a 500 mA ISL85415 switching
4. A-Star 32U4 Mini Page 19 of 62
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
step-down (buck) converter from Renesas (which acquired Intersil). (We also make a standalone
regulator [https://www.pololu.com/product/2843] based on this integrated circuit.)
The A-Star 32U4 Mini SV (ac02f) is the newest version of the Mini SV, and it can be powered from a
5 V to 40 V external source. The input voltage is regulated to 5 V by an 800 mA ISL85418 switching
step-down converter, which is part of the same family as the ISL85415. (We also make a standalone
regulator based on the 1 A version [https://www.pololu.com/product/2831] of this integrated circuit.)
The graphs below show the efficiency of the two SV versions, where efficiency is defined as
(Power out)/(Power in):
Typical efficiency of the regulator on
the A-Star 32U4 Mini SV (original ac02c
version).
Typical efficiency of the regulator on
the A-Star 32U4 Mini SV (newer ac02f
version).
The A-Star’s components, including the microcontroller and LEDs, draw 30 mA to 40 mA in typical
applications. The rest of the regulator’s achievable output current, which depends on input voltage as
well as ambient conditions, can be used to power other devices. The green lines in the graph below
show the output currents where the regulator’s output voltage drops below 4.75 V. These currents
are close to the limits of the regulator’s capability and generally cannot be sustained for long periods;
under typical operating conditions, a safe limit for the maximum continuous regulator output current is
approximately 500 mA for ac02c and 800 mA for ac02f.
4. A-Star 32U4 Mini Page 20 of 62
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
The dropout voltage of a step-down regulator is defined as the minimum amount by which the input
voltage much exceed the regulator’s target output voltage in order to assure the target output can be
achieved. As can be seen in the graphs below for the two versions of the Mini SV, the dropout voltages
of the Mini SV’s regulators increase approximately linearly with the output current. For light loads
where the dropout voltage is small, the boards can operate down to 5 V. However, for larger loads, the
dropout voltage should be taken into consideration when selecting a power supply; operating above
6 V will ensure the full output current is available.
Typical dropout voltage of the 5V
regulator on the A-Star 32U4 Mini SV
(original ac02c version).
Typical dropout voltage of the 5V
regulator on the A-Star 32U4 Mini SV
(newer ac02f version).
4. A-Star 32U4 Mini Page 21 of 62
• A-Star 32U4 Mini ULV dimension diagram [https://www.pololu.com/file/0J781/ac02a-
dimensions.pdf] (274k pdf)
• A-Star 32U4 Mini LV dimension diagram [https://www.pololu.com/file/0J782/ac02b-
dimensions.pdf] (292k pdf)
• A-Star 32U4 Mini SV dimension diagram [https://www.pololu.com/file/0J1512/a-star-32u4-mini-sv-
dimensions.pdf] (208k pdf)
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
Note: Batteries can have much higher voltages than their nominal voltages when fully
charged, so be careful with nominal voltages above 24 V. A 36 V battery is not appropriate
for this product.
4.5. A-Star 32U4 Mini schematic and dimensions
Schematic diagram
The schematic diagram for the A-Star 32U4 Minis is available as a PDF: A-Star 32U4 Mini schematic
diagram [https://www.pololu.com/file/0J780/a-star-32u4-mini-schematic.pdf] (243k pdf).
Dimension diagrams
Dimension diagrams for the A-Star 32U4 Minis are available as PDFs:
4. A-Star 32U4 Mini Page 22 of 62
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
5. A-Star 32U4 Prime
A-Star 32U4 Prime LV microSD. A-Star 32U4 Prime SV microSD.
5.1. A-Star 32U4 Prime configurations
The A-Star 32U4 Prime LV (blue solder mask, 2 V to 16 V operating voltage range) and SV (green
solder mask, 5 V to 36 V operating voltage range) are each available in five different versions, allowing
you to choose the right amount of customizability and features for your application. (The pictures here
show the LV board, but they also reflect the corresponding SV configurations.)
A-Star 32U4 Prime LV (SMT Components Only) A-Star 32U4 Prime SV (SMT Components Only)
This version comes with all surface-mount components soldered to
the board (including the USB connector, pushbuttons, and power
switch), except for the microSD card connector and level shifters,
which are not populated. It comes with no headers, DC barrel jack, or
buzzer.
A-Star 32U4 Prime LV microSD (SMT Components Only) A-Star 32U4 Prime SV microSD (SMT Components Only)
This version adds the microSD card connector and level shifters, but
is otherwise identical to the preceding version.
5. A-Star 32U4 Prime Page 23 of 62
• four standard Arduino female headers (one 1×6, two 1×8,
one 1×10) on the outermost pins, matching an Arduino
Leonardo or Uno R3
• 6-pin ISP header (2×3 male)
• DC barrel jack
• buzzer
• 1×2 low-profile male header for buzzer jumper (shorting
block included)
• 1×2 low-profile male header for battery level jumper (shorting
block included)
• LCD connector (2×7 male header)
• 3.5 mm screw terminal block
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
A-Star 32U4 Prime LV A-Star 32U4 Prime SV
This version comes with all surface-mount components except for the
microSD card connector and level shifters. These through-hole parts
are also soldered in:
A-Star 32U4 Prime LV microSD A-Star 32U4 Prime SV microSD
This version adds the microSD card connector and level shifters, as
well as a third 1×2 low-profile male header and shorting block for the
microSD CS jumper, but is otherwise identical to the preceding
version.
A-Star 32U4 Prime LV microSD with LCD A-Star 32U4 Prime SV microSD with LCD
This version is the same as the preceding version, but it comes with a
8×2 character LCD [https://www.pololu.com/product/356] (with 2×7 female
header soldered), and the following parts are additionally soldered to
the board:
5. A-Star 32U4 Prime Page 24 of 62
• four standard Arduino female headers [https://www.pololu.com/category/50/0.1-2.54-mm-female-
headers] (one 1×6, two 1×8, one 1×10)
• four 1×25 male header strips [https://www.pololu.com/product/965]
• 2×3 male header (ISP)
• 2×7 male header (LCD)
• 2×7 female header [https://www.pololu.com/product/1027] (LCD)
• 1×2 female header [https://www.pololu.com/product/1012]
• 1×10 low-profile male header strip
• three low-profile shorting blocks
• buzzer
• DC barrel jack [https://www.pololu.com/product/1139]
• 2-pin 3.5 mm screw terminal block [https://www.pololu.com/product/2444]
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
For users wishing to populate some or all of the through-hole parts on an SMT-only version of
the A-Star 32U4 Prime (or to add more functionality to another version), we offer an A-Star Prime
accessory pack [https://www.pololu.com/product/3110] that contains these items:
The 8×2 character LCD [https://www.pololu.com/product/356] is also available separately.
Note: The A-Star 32U4 Prime does not come with a microSD card. The microSD
connector is an optional feature that can be used to increase the data storage available
to the board, but it is not required.
5.2. A-Star 32U4 Prime pinout and components
Pinout
5. A-Star 32U4 Prime Page 25 of 62
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
This diagram identifies the I/O and power pins on the A-Star 32U4 Prime (LV and SV versions);
it is also available (along with the power distribution diagram below) as a printable PDF
[https://www.pololu.com/file/0J1653/a-star-32u4-prime-pinout-power.pdf] (1MB pdf). For more information about
the ATmega32U4 microcontroller and its peripherals, see Atmel’s ATmega32U4 documentation.
The outermost rows of pins of the A-Star 32U4 Prime correspond to the pins on an Arduino Leonardo,
and each is duplicated on a second inner row for more convenient access. Printed on the A* circuit
board are indicators that you can use to quickly identify each I/O pin’s capabilities: a triangle by the
inner through hole means the pin can be used as an analog input, and a square wave symbol next to
the hole pair means the pin can be used as a PWM output.
Additional rows of power and ground pins also run along most of the I/O pins to allow easy connections
to devices like sensors and servos. The innermost pin is ground, and the second-innermost is power,
which is not connected by default. The power pins are split into three buses: one bus for pins A0–A4
along the bottom, and two buses along the top, split between digital pins 7 and 8. You can configure
each bus separately by using a jumper to connect it to a voltage source of your choice. (Access points
for 5V and VIN are adjacent to the A0–A4 bus, so it can be conveniently connected to either supply
with a shorting block.)
5. A-Star 32U4 Prime Page 26 of 62
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
Compatibility with Arduino shields and accessories
The A-Star 32U4 Prime matches the Arduino Leonardo and the Arduino Uno R3 in the shape
of its circuit board and the arrangement of its pins. Furthermore, it uses the same ATmega32U4
microcontroller as the Leonardo, running at the same voltage and frequency, so the A* should
generally work with any shield or accessory that is compatible with the Leonardo (including our Zumo
Robot for Arduino [https://www.pololu.com/product/2510]).
LEDs
The A-Star 32U4 Prime has five indicator LEDs.
A blue power LED located next to the power switch indicates when the onboard regulator is active (an
external power supply is connected and the switch is turned on).
A green power LED next to the USB connector indicates when the USB bus voltage (VBUS) is
present.
A yellow user LED is connected to Arduino pin 13, or PC7. You can drive this pin high in a user
program to turn this LED on. The A-Star 32U4 Bootloader [https://www.pololu.com/docs/0J61/9] fades this
LED on and off while it is waiting for a sketch to be loaded.
A green user LED is connected to PD5 and lights when the pin is driven low. While the board is
running the A-Star 32U4 Bootloader or a program compiled in the Arduino environment, it will flash
this LED when it is transmitting data via the USB connection.
5. A-Star 32U4 Prime Page 27 of 62
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
A red user LED is connected to Arduino pin 17, or PB0, and lights when the pin is driven low. While
the board is running the A-Star 32U4 Bootloader or a program compiled in the Arduino environment, it
will flash this LED when it is receiving data via the USB connection.
The AStar32U4Prime library contains functions that make it easier to control the three user LEDs. All
three user LED control lines are also LCD data lines, so you will see them flicker when you update the
LCD. The green and red user LEDs also share I/O lines with pushbuttons (see below).
Pushbuttons
The A-Star 32U4 Prime has four pushbuttons: a reset button next to the power switch and three
user pushbuttons located along the right edge of the board. The user pushbuttons, labeled A, B, and
C, are on Arduino pin 14 (PB3), PD5, and Arduino pin 17 (PB0), respectively. Pressing one of these
buttons pulls the associated I/O pin to ground through a resistor.
The three buttons’ I/O lines are also used for other purposes: pin 14 is MISO on the SPI interface, PD5
and pin 17 control the green and red user LEDs, and all three pins are LCD data lines. Although these
uses require the pins to be driven by the AVR (or SPI slave devices in the case of MISO), resistors
in the button circuits ensure that the A-Star will not be damaged even if the corresponding buttons
are pressed at the same time, nor will SPI or LCD communications be disrupted. The functions in the
AStar32U4Prime library take care of configuring the pins, reading and debouncing the buttons, and
restoring the pins to their original states.
Note that button A cannot be used if the microSD CS pin is tied to ground with a jumper (see microSD
card connector and level shifters below).
5. A-Star 32U4 Prime Page 28 of 62
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
Buzzer
The assembled versions of the A-Star 32U4 Prime come
with a buzzer that can be used to generate simple
sounds and music. The buzzer is not present on the
SMT-only versions, but the buzzer driver circuit is still
populated, allowing you to solder in your own buzzer or
speaker. The stock buzzer is available as part of the A-
Star 32U4 Prime accessory pack [https://www.pololu.com/
product/3110].
A through-hole jumper next to the buzzer provides a way
to connect the buzzer input to digital pin 6 (which also
serves as OC4D, a hardware PWM output from the
AVR’s 10-bit Timer4). If you alternate between driving the
buzzer pin high and low at a given frequency, the buzzer
will produce sound at that frequency. You can play notes
and music with the buzzer using functions in the
AStar32U4PrimeBuzzer library.
Buzzer and CS-GND jumpers on the A-
Star 32U4 Prime.
microSD card connector and level shifters
Some versions of the A-Star 32U4 Prime include an onboard microSD card connector that enables
the microcontroller to read from and write to microSD memory cards. The card socket is connected to
the SPI interface on the ATmega32U4 through level-shifting circuits, allowing the 5 V microcontroller
to safely communicate with standard 3.3 V SD cards. DI, DO, and SCLK on the card are connected to
MOSI, MISO, and SCK on the AVR, respectively. The Arduino SD library [http://arduino.cc/en/Reference/
SD] can be used to access the file system on an inserted microSD card.
A fourth pin, CS (chip select), must be held low to enable communication with the microSD card in
SPI mode. This can be done by connecting one of the microcontroller’s I/O lines to CS (the SD library
examples use digital pin 4 by default) and driving the pin low to select the microSD card. The CS line
also controls whether the output of the DO level shifter is enabled: when CS is high, the level shifter
places its output into a high-impedance state, allowing the MISO line to be driven by other SPI slave
devices, used as an LCD control line, or read as a pushbutton input.
Alternatively, you can tie CS low more easily by shorting it to the adjacent ground pin, causing the
microSD card to always be selected and the DO level shifter output to always be enabled. This allows
you to avoid using an extra I/O line and making an additional connection, but the DO level shifter
output will override the pushbutton on the same pin and prevent any presses on button A from being
detected. You should remove the CS-GND jumper when no microSD card is present, as asserting CS
5. A-Star 32U4 Prime Page 29 of 62
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
with no microSD card inserted can put the DO level shifter output into an undefined state.
The CD (card detect) pin next to the connector can be used to detect the presence of a microSD card:
it is floating when a card is inserted and shorted to ground when a card is not present. Two additional
SD data lines (DAT1 and DAT2), which are unused in SPI mode, are also broken out for advanced
uses.
LCD
The A-Star 32U4 Prime has a mounting location for a 2×7 header where you can connect a character
LCD with the common HD44780 parallel interface [https://www.pololu.com/file/0J71/DMC50448N-AAE-
AD.pdf] (109k pdf). The A* is optionally available with a male header installed here and an 8×2
character LCD [https://www.pololu.com/product/356] (with corresponding female header) included; on
other versions, you can add your own display using the connectors of your choice. A larger LCD can
be connected with a ribbon cable [https://www.pololu.com/product/973] and optionally a shrouded box
header [https://www.pololu.com/product/898].
The LCD control lines are broken out to a column of through holes next to the LCD connector,
labeled on the back side of the board. By default, some of these are connected to I/O lines from the
ATmega32U4 to allow control of the LCD in 4-bit mode, but you can remap the connections by cutting
the surface-mount jumpers indicated in the picture below and making new connections between I/O
lines and LCD control pins.
5. A-Star 32U4 Prime Page 30 of 62
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
The AStar32U4PrimeLCD library provides functions to display data on a connected LCD. It is designed
to gracefully handle alternate use of the LCD data lines by only changing pin states when needed for
an LCD command, after which it will restore them to their previous states. This allows the LCD data
lines to be used for other functions (such as pushbutton inputs and LED drivers).
Other connectors
The A-Star 32U4 includes a USB Micro-B connector that can be used to connect to a computer’s
USB port via a USB A to Micro-B cable [https://www.pololu.com/product/2072] (not included). The USB
connection can be used to transmit and receive data from the computer, and a preloaded USB
bootloader makes it possible to program the board over USB. The USB connection can also provide
power to the A-Star.
5. A-Star 32U4 Prime Page 31 of 62
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
The board also has a 6-pin ISP header that allows it to be programmed with an external programmer,
such as our USB AVR programmer v2.1 [https://www.pololu.com/product/3172]. Pin 1 of the header is
indicated with a small dot and has an octagonal shape. Three of the pins on this header can be used
as an SPI interface or as general-purpose digital I/O, as labeled on the back side of the board and
shown in the pinout diagram. In the Arduino environment, you can refer to these three pins using either
their pin numbers or the names of their SPI functions (which are defined as aliases); for example,
digitalRead(15) and digitalRead(SCK) are equivalent. If you have an SMT-only version of the A-
Star, it is possible to solder a shrouded box header [https://www.pololu.com/product/854] to this location;
the outlines on the board indicate the correct connector orientation.
Power
The A-Star 32U4 Prime can either be powered directly from the USB 5 V supply or from an external
voltage source, which is regulated to 5 V by its onboard switching regulator. The slide switch on A*
controls whether the external source is connected to the input of the regulator, providing a convenient
way to switch off external power to the A-Star without unplugging any connections. The adjacent set
of three pins provides a place to connect your own power switch: to enable external power, connect
the middle pin to ground (accessible through the upper pin).
When the A-Star is connected to a computer via USB, it will receive 5V power even when
the power switch is off. This can be useful if you want to upload or test a program without
drawing external power (to avoid draining a battery pack, for example).
The board’s power selection circuit uses the TPS2113A power multiplexer [https://www.pololu.com/
product/2596] from Texas Instruments to choose whether its 5 V supply is sourced from USB or an
external supply via the regulator, allowing both sources to be connected at the same time and enabling
the A-Star to safely and seamlessly transition between them. The TPS2113A is configured to select
external power unless the regulator output falls below about 4.5 V. If this happens, it will select the
higher of the two sources, which will typically be the USB 5 V bus voltage if the A* is connected to
USB. The currently selected source is indicated by the STAT pin in the middle of the board; this pin
is an open-drain output that is low if the external power source is selected and high-impedance if the
USB supply is selected. The current limit of the TPS2113A is set to about 1.9 A. For more information
about the power multiplexer, see the TPS2113A datasheet [https://www.pololu.com/file/0J771/tps2113a.pdf]
(1MB pdf).
In some situations, it might be undesirable for the A-Star 32U4 Prime to draw power from an external
source when it is connected to USB, even if the power switch is left on. If this is the case, the regulator
can be disabled by driving the regulator shutdown pin, SHDN, high; this shuts down the regulator and
causes the power mux to fall back to USB power. For example, this could allow a battery-powered
system to automatically turn off the regulator while it is connected to a computer.
5. A-Star 32U4 Prime Page 32 of 62
• LV: 2 V to 16 V (see Section 5.3 for regulator details)
• SV: 5 V to 36 V (see Section 5.4 for regulator details)
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
The input voltage range of the regulator depends on the particular version of the A-Star 32U4 Prime:
Reverse-protected and switched power inputs: The connection points labeled “Power In” are
power inputs with reverse-voltage protection that are controlled by the power switch. These are the
recommended pins to use when connecting an external power supply because they allow the A-Star’s
reverse-voltage protection circuit to help prevent it from being damaged by accidentally-reversed
power connections, and they enable the A-Star’s power switch to disconnect the supply.
A number of different connectors can be soldered to the board and used to provide power, including
a DC barrel jack [https://www.pololu.com/product/1139] or a 3.5 mm screw terminal block
[https://www.pololu.com/product/2444] (the assembled versions of the board come with one or both of
these populated).
Warning: You must never connect different power sources to multiple Power In locations at
the same time, as doing so will create a short between the supplies.
VIN power output (or alternative input): When power is supplied through the Power In pins, the
VIN pin can be used as an output to supply reverse-protected and switched power to other devices.
Alternatively, the external supply can be connected directly between VIN and GND, bypassing the
reverse-voltage protection and power switch.
5V power output: This pin provides access to the board’s 5 V supply, which comes from either the
USB 5 V bus voltage or the onboard switching regulator, depending on which power sources are
connected and enabled. Note that some of the available current on the 5 V line is used by the board
itself (typically at least 30 mA, but potentially much more if peripherals like the buzzer are active) or
used to provide current for the GPIO pins or 3.3 V regulator (see below).
3V3 power output: This pin gives access to the output of the onboard 3.3 V linear regulator. The
microSD card, if present, draws power from the 3.3 V line; the remainder (up to a few hundred
milliamps) is available for powering external circuits or devices.
When the A-Star 32U4 Prime is being powered through Power In, the sum of the 5V output current,
3V3 output current, GPIO output current, and current used by the board itself should not exceed the
maximum current that the switching regulator can provide.
In a battery-powered application, it might be useful for the A-Star to monitor the battery’s voltage
level. The BATLEV pin provides access to a voltage divider that outputs a fraction of the VIN
5. A-Star 32U4 Prime Page 33 of 62
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
voltage (one-third on the LV, one-eighth on the SV), and this voltage can be read by connecting it
to the adjacent analog pin 1 (A1) (or another analog input). The readBatteryMillivoltsLV() and
readBatteryMillivoltsSV() functions in the AStar32U4Prime library can be used to determine the
battery voltage from this reading.
Battery level jumper on the A-Star
32U4 Prime.
5.3. A-Star 32U4 Prime LV regulator
There have been two versions of the A-Star 32U4 Prime LV: the original ac03b version
and its replacement, the newer ac03e version. The ac03e version uses an improved 5 V
regulator that can deliver more current (1.8 A vs 1.5 A) and operates from 2 V to 16 V
instead of 2.7 V to 11.8 V. The easiest way to distinguish between the two versions is via
the silkscreen on the bottom side of the board, above and to the left of the Pololu logo,
where the original version is labeled ac03b and the new version is labeled ac03e. See
the picture below for a side-by-side comparison:
5. A-Star 32U4 Prime Page 34 of 62
• A 4-cell battery holder, which might have a 6 V output with fresh alkalines or a 4.0 V output
with partially discharged NiMH cells, can be used to power this A*.
• A disposable 9 V battery powering the board can be discharged to under 3 V instead of
cutting out at 6 V, as with typical linear or step-down regulators.
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
A-Star 32U4 Prime LV: comparison of original ac03b version (left) to newer ac03e version
(right).
The A-Star 32U4 Prime LV (ac03b) is the original version of the Prime LV, and it can be powered from
a 2.7 V to 11.8 V external source. The input voltage is regulated to 5 V by a 1.5 A TPS63061 switching
step-up step-down (buck-boost) converter from Texas Instruments. (We also make a standalone
regulator [https://www.pololu.com/product/2123] based on this integrated circuit.)
The A-Star 32U4 Prime LV (ac03e) is the newest version of the Prime LV, and it can be powered
from a 2 V to 16 V external source (though it requires at least 3 V at start-up). The input voltage
is regulated to 5 V by a 1.8 A TPS630701 switching step-up step-down (buck-boost) converter from
Texas Instruments. (We also make a standalone regulator [https://www.pololu.com/product/2836] based
on this integrated circuit.)
For both versions of the Prime LV, the regulators’ flexibility in input voltage is especially well-suited
for battery-powered applications in which the battery voltage begins above 5 V and drops below 5
V as the battery discharges. Without the typical restriction on the battery voltage staying above 5 V
throughout its life, a wider range of battery types can be considered. For example:
The graphs below show the efficiency of the two LV versions, where efficiency is defined as
(Power out)/(Power in):
5. A-Star 32U4 Prime Page 35 of 62
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
Typical efficiency of the 5 V regulator
on the A-Star 32U4 Prime LV (original
ac03b version).
Typical efficiency of the 5 V regulator
on the A-Star 32U4 Prime LV.
The A-Star’s components, including the microcontroller and LEDs, draw 30 mA to 40 mA in typical
applications. The rest of the regulator’s achievable output current, which depends on input voltage as
well as ambient conditions, can be used to power other devices. Maximum ouput currents for both
versions of the A-Star Prime LVs are shown in the graph below. These currents are close to the
limits of the regulators’ capabilities and generally cannot be sustained for long periods; under typical
operating conditions, a safe limit for the maximum continuous regulator output current is approximately
1.5 A for ac03b and 1.8 A for ac03e.
5. A-Star 32U4 Prime Page 36 of 62
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
Note that the startup current for the regulator on the A-Star 32U4 Prime LV (ac03e) is
limited to approximately 700 mA, and currents in excess of this are only available after
the output has finished rising to 5 V.
5.4. A-Star 32U4 Prime SV regulator
The A-Star 32U4 Prime SV can be powered from a
5 V to 36 V external source. The input voltage is
regulated to 5 V by a 1 A ISL85410 switching step-
down (buck) converter from Intersil. (We also make a
standalone regulator [https://www.pololu.com/product/
2831] based on this integrated circuit.)
As shown in the left graph below, the SV’s switching
regulator has an efficiency – defined as
(Power out)/(Power in) – of 80% to 95% for most
combinations of input voltage and load.
The A-Star’s components, including the microcontroller and LEDs, draw 30 mA to 40 mA in typical
applications (without the buzzer, microSD card, or an LCD). The rest of the regulator’s achievable
output current, which depends on input voltage as well as ambient conditions, can be used to power
other devices. The right graph above shows the output currents where the regulator’s output voltage
drops below 4.75 V. These currents are close to the limits of the regulator’s capability and generally
cannot be sustained for long periods; under typical operating conditions, a safe limit for the maximum
continuous regulator output current is approximately 1 A.
The dropout voltage of a step-down regulator is defined as the minimum amount by which the input
voltage much exceed the regulator’s target output voltage in order to assure the target output can
be achieved. As can be seen in the graph below, the dropout voltage of the Prime SV’s regulator
increases approximately linearly with the output current. For light loads where the dropout voltage is
5. A-Star 32U4 Prime Page 37 of 62
• A-Star 32U4 Prime LV schematic diagram (ac03e) [https://www.pololu.com/file/0J1648/a-
star_32u4_prime_lv_schematic_diagram.pdf] (382k pdf)
• A-Star 32U4 Prime LV schematic diagram (ac03b) [https://www.pololu.com/file/0J846/a-
star_32u4_prime_lv_schematic_diagram.pdf] (686k pdf)
• A-Star 32U4 Prime SV schematic diagram [https://www.pololu.com/file/0J856/a-
star_32u4_prime_sv_schematic_diagram.pdf] (1MB pdf)
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
small, the board can operate down to 5 V. However, for larger loads, the dropout voltage should be
taken into consideration when selecting a power supply; operating above 6 V will ensure the full output
current is available.
Typical dropout voltage of the 5 V regulator
on the A-Star 32U4 Prime SV.
Note: Although the ISL85410 is rated for a maximum operating input voltage of 36 V, it
is not appropriate to power the Prime SV with a 36 V battery, as battery voltages can
be much higher than nominal voltages when they are charged. The maximum nominal
battery voltage we recommend is 24 V, and if you approach that limit, you should take extra
precautions to prevent LC voltage spikes from damaging the board (see this application
note [https://www.pololu.com/docs/0J16] for more information). A good practice is to ensure that
the A-Star’s power switch is off before connecting it to a voltage source.
5.5. A-Star 32U4 Prime schematic and dimensions
Schematic diagram
The schematic diagrams for the A-Star 32U4 Primes are available as PDFs:
5. A-Star 32U4 Prime Page 38 of 62
• A-Star 32U4 Prime LV dimensions diagram (ac03e) [https://www.pololu.com/file/0J1650/a-
star-32u4-prime-lv-dimensions.pdf] (2MB pdf)
• A-Star 32U4 Prime LV dimensions diagram (ac03b) [https://www.pololu.com/file/0J1144/a-
star-32u4-prime-lv-dimension-diagram.pdf] (4MB pdf)
• A-Star 32U4 Prime SV dimensions diagram [https://www.pololu.com/file/0J1145/a-
star-32u4-prime-sv-dimension-diagram.pdf] (4MB pdf)
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
Dimension diagram
The dimension diagrams for the A-Star 32U4 Primes are available as PDFs:
5.6. A-Star 32U4 Prime demo program
The A-Star 32U4 Prime LV microSD with LCD [https://www.pololu.com/product/4009] and A-Star 32U4
Prime SV microSD with LCD [https://www.pololu.com/product/3115] each come preloaded with a program
that demonstrates many of its features and allows you to test that it is working correctly, and this demo
program will run when you first turn on the board.
The demo program uses the standard A-Star 32U4 Prime LCD pins, buzzer pin, button pins,
microSD pins, and pin 4. To avoid damage or improper operation, if you have shields or
other electronics connected, make sure they do not use those pins in a conflicting way.
When the demo program starts, you should see the words “A-Star
Prime” and then “Demo Program” appear. If you do not see any text
on the LCD, you may need to adjust the contrast potentiometer. The
demo program is not usable without an LCD. If a buzzer is soldered in
and the buzzer shorting block is installed, you should also hear a beep
when the demo program starts.
Buzzer and CS-GND jumpers
on the A-Star 32U4 Prime.
5. A-Star 32U4 Prime Page 39 of 62
• Keyboard: This demo makes the A-Star act as a two-
key keyboard. If the A-Star is connected to a computer
via USB, you can press button A or button C to send the
corresponding key presses to the computer.
• LEDs: Blinks the red, green, and yellow user LEDs.
• Music: Plays an adaptation of J. S. Bach’s Fugue in D
Minor for microcontroller and piezo, while scrolling a text
display. This demonstrates the ability for the A-Star to
play music in the background.
• Power: Displays information about the board’s power
sources. The top line displays the measured voltage on
VIN, in millivolts. For this measurement to work, a jumper
should be installed to connect A1 to BATLEV. The board
should also be powered from one of the connection
points labeled “Power In”, and the power switch must be
in the “On” position, or else the reading will be 0 mV.
The bottom line displays “USB=Y” if power from the USB
connector is detected and “USB=N” if it is not detected.
• SD card: This demo communicates with the microSD
card to get the size of the card’s main partition and then
displays it on the LCD, in units of megabytes. To use
this part of the program, you will need to install a jumper
between GND and CS. (You will also need to have a
version of the board with a microSD card socket and
you will need to insert a formatted microSD card into the
socket.) While the CS-GND jumper is installed, button
A will not work. Also, if the CS-GND jumper is installed
and the microSD card is not inserted, the program might
detect spurious presses on button A. To avoid these
issues, you can connect CS to pin 4 using a male-female
jumper wire [https://www.pololu.com/category/67/male-female-
premium-jumper-wires] instead of installing the jumper.
• Serial: This demo uses the A-Star’s virtual USB serial
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
After the program has started, press the B button to proceed to the
main menu. Press C or A to scroll forward or backward through the
menu, and press B to make a selection or to exit one of the demos.
There are six demos accessible from the menu:
Battery level jumper on the A-
Star 32U4 Prime.
5. A-Star 32U4 Prime Page 40 of 62
port to send and receive data from the computer. You can connect to the A-Star’s serial port
using a standard terminal program or by using the Serial Monitor feature from the Tools menu
in the Arduino IDE. Any characters you send to the A-Star will be displayed on the LCD, and
information about the received characters will be sent back to the computer. If you press the
A or C buttons, a message will be sent to the computer saying they were pressed.
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
The A-Star 32U4 Prime versions with included LCDs also come with the buzzer and battery voltage
sensing jumpers installed by default and the contrast potentiometer adjusted properly, so the LCD,
buzzer, and voltage sensing should work by default.
The source code of the demo program is included with the AStar32U4Prime library, so you can
compile it and load it onto any A-Star 32U4 Prime using the Arduino IDE.
5. A-Star 32U4 Prime Page 41 of 62
1. Download the A-Star Windows Drivers [https://www.pololu.com/file/0J1240/a-star-
windows-1.3.0.0.zip] (7k zip) and extract the ZIP file to a temporary folder on your computer.
(These files are also available in the “drivers” directory from the A-Star repository on
GitHub [https://github.com/pololu/a-star].)
2. Open the “a-star-windows” folder. Right-click on “a-star.inf” and select “Install”.
3. Windows will ask you whether you want to install the drivers. Click “Install” (Windows 10, 8,
7, and Vista) or “Continue Anyway” (Windows XP).
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
6. Getting started
6.1. Installing Windows drivers
If you use Windows XP, you will need to have either Service Pack 3 or Hotfix KB918365
installed before installing the A-Star drivers. Some users who installed the hotfix have
reported problems that were solved by upgrading to Service Pack 3, so we recommend
Service Pack 3 over the hotfix.
Before you connect your Pololu A-Star 32U4 (or another of our 32U4 family of boards) to a computer
running Microsoft Windows, you should install its drivers:
6. Getting started Page 42 of 62
4. Windows will not tell you when the installation is complete, but it should be done after a few
seconds.
5. Connect the device to your computer’s USB port.
6. When the “Found New Hardware Wizard” is displayed, select “No, not this time” and click
“Next”.
7. On the second screen of the “Found New Hardware Wizard”, select “Install the software
automatically” and click “Next”.
8. Windows XP will warn you again that the driver has not been tested by Microsoft and
recommend that you stop the installation. Click “Continue Anyway”.
9. When you have finished the “Found New Hardware Wizard”, click “Finish”.
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
Windows 10, Windows 8, Windows 7, and Windows Vista users: After installing the drivers, your
computer should automatically recognize the device when you connect it via USB. No further action
from you is required. However, the first time you connect an A-Star device to your computer, Windows
will take several seconds to recognize the device and configure itself properly. The first time you
program the device, Windows will again take several seconds to recognize the A-Star USB bootloader,
and this could cause the programming operation to fail the first time. Also, Windows will need to re-
recognize the device and the bootloader if you connect the board to another USB port that it has not
been connected to before.
Windows XP users: After installing the drivers, you will need to follow steps 5–9 for each new A-Star
device you connect to your computer. You will also need to follow these steps the first time you attempt
to program the device in order to make Windows recognize the bootloader, and when you connect the
device to a different USB port that it has not been connected to before.
6. Getting started Page 43 of 62
1. In the Arduino IDE, open the File menu
(Windows/Linux) or the Arduino menu
(macOS) and select “Preferences”.
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
COM port details
After installing the drivers and plugging in an A-Star, in the “Ports (COM & LPT)” category of the Device
Manager, you should see a COM port for the A-Star’s running sketch named “Pololu A-Star 32U4”.
You might see that the COM port is named “USB Serial Device” in the Device Manager instead of
having a descriptive name. This can happen if you are using Windows 10 or later and you plugged
the A-Star into your computer before installing our drivers for it. In that case, Windows will set up your
A-Star using the default Windows serial driver (usbser.inf), and it will display “USB Serial Device” as
the name for the port. The port will still be usable, but it will be hard to tell if it is the right one because
of the generic name shown in the Device Manager. We recommend fixing the names in the Device
Manager by right-clicking on each “USB Serial Device” entry, selecting “Update Driver Software…”,
and then selecting “Search automatically for updated driver software”. Windows should find the drivers
you already installed, which contain the correct name for the port.
If you are using Windows 10 or later and choose not to install the drivers, the A-Star will still be
usable. To tell which “USB Serial Device” in your Device Manager is the A-Star, double-click on each
one and look at the “Hardware Ids” property in the “Details” tab. An A-Star running a sketch will
have the ID USB\VID_1FFB&PID_2300&MI_00 , while an A-Star in bootloader mode will have the ID USB\
VID_1FFB&PID_0101 .
If you want to change the COM port numbers assigned to your A-Star, you can do so using the Device
Manager. Double-click a COM port to open its properties dialog, and click the “Advanced…” button in
the “Port Settings” tab.
6.2. Programming using the Arduino IDE
Our 32U4 family of boards can be programmed from the
popular Arduino integrated development environment
(IDE). The Arduino IDE is a cross-platform, open source
application that integrates a C++ code editor, the GNU
C++ compiler, and a program upload utility. To get started
programming your device with the Arduino IDE (version
1.6.4 or later), follow these steps:
Download the Arduino IDE from the Arduino Download
page [http://arduino.cc/en/Main/Software], install it, and start
it.
Programming the A-Star 32U4 from the
Arduino IDE.
6. Getting started Page 44 of 62
2. In the Preferences dialog, find the “Additional Boards Manager URLs” text box (highlighted in
the picture below). Copy and paste the following URL into this box:
https://files.pololu.com/arduino/package_pololu_index.json
If there are already other URLs in the box, you can either add this one separated by a comma
or click the button next to the box to open an input dialog where you can add the URL on a
new line.
3. Click the “OK” button to close the Preferences dialog.
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
Adding a Boards Manager index for Pololu boards in the Arduino IDE’s Preferences dialog.
6. Getting started Page 45 of 62
4. In the Tools > Board menu, select “Boards Manager…” (at the top of the menu).
5. In the Boards Manager dialog, search for “Pololu A-Star Boards”.
6. Select the “Pololu A-Star Boards” entry in the list, and click the “Install” button.
7. After the installation finishes, click the “Close” button to close the Boards Manager dialog.
8. In the Tools > Board menu, select the “Pololu A-Star 32U4” entry. If you do not see your
device listed in the Board menu, try restarting the Arduino IDE.
9. In the Tools > Port menu, select the port for the device. On Windows you can determine
what COM port the device is assigned to by looking at the “Ports (COM & LPT)” section of
the Device Manager. On Linux, the port name will begin with “/dev/ttyACM”. On Mac OS X,
the port name will begin with “/dev/tty.usbmodem”.
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
Selecting the Pololu A-Star 32U4 in the Boards menu.
6. Getting started Page 46 of 62
10. Open up the “Blink” Arduino example, which can be found under File > Examples >
01.Basics > Blink. The code in this example will blink the yellow LED. When you select the
Blink example, a new Arduino IDE window will open up. It is OK to close the first window.
11. Press the “Upload” button to compile the sketch and upload it to the device. If everything goes
correctly, you will see the message “Done uploading” appear near the bottom of the window.
If you are using Windows and you have not previously programmed an A-Star device on this
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
Windows 10 Device Manager showing the A-Star’s virtual COM port.
Selecting the Blink example in the Arduino IDE.
6. Getting started Page 47 of 62
USB port, then Windows might take several seconds to recognize the A-Star bootloader. The
bootloader times out after 8 seconds and goes back to running the sketch, so the upload
might fail if Windows does not recognize it quickly enough. If this happens, try again. If you
are using Windows XP and have not programmed an A-Star on this USB port, you will have to
go through the Found New Hardware Wizard again as described in the previous section, but
the second time you try to upload it should work. If the Arduino IDE has trouble connecting to
the port or using it, try unplugging the device, closing any programs that might be using the
serial port, restarting the Arduino IDE, and then plugging the device back in.
12. If you uploaded the Blink sketch, then the yellow LED should be blinking once every two
seconds. However, we ship some A-Stars with that same example already programmed onto
it, so you might not be convinced that anything has changed. Try changing the delay values
in the sketch to something else and uploading again to see if you can change the speed of
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
Uploading a sketch to the A-Star using the Arduino IDE.
6. Getting started Page 48 of 62
the LED.
• The Arduino IDE has many examples [http://arduino.cc/en/Tutorial/HomePage] that can run on A-
Stars.
• The Arduino website has a Language Reference [http://arduino.cc/en/Reference/HomePage], a
wiki called the The Arduino Playground [http://playground.arduino.cc/], and other resources.
• The A-Star 32U4 boards are similar to the Arduino Leonardo [https://www.pololu.com/product/
2192] and Arduino Micro [https://www.pololu.com/product/2188], so you can search the Internet
for relevant projects that use one of those boards.
• The Related Resources section lists many more resources.
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
The A-Star 32U4 boards are similar enough to the Arduino Leonardo that you do not
actually have to install the add-on. If you want to, you can just select the “Arduino
Leonardo” board in the Arduino IDE. Note that if you upload a sketch to the device this
way, your computer will then recognize it as a Leonardo (for example, its entry in the
Windows Device Manager will display “Arduino Leonardo”).
After you succeed in programming your device from the Arduino IDE, there are many resources you
can use to learn more:
6.3. Programming using avr-gcc and AVRDUDE
This section explains how to program our 32U4 family of boards using the avr-gcc toolchain and
AVRDUDE. This section is intended for advanced users who do not want to use the Arduino IDE as
described in the previous section.
Getting the prerequisites
If you are using Windows, we recommend downloading WinAVR [http://winavr.sourceforge.net/], which
contains the avr-gcc toolchain and a command-line utility called AVRDUDE [http://www.nongnu.org/
avrdude/] that can be used to upload programs to the A-Star bootloader. If the version of GNU Make
that comes with WinAVR crashes on your computer, we recommend using the Pololu version of GNU
Make [https://github.com/pololu/make/releases].
If you are using Mac OS X, we recommend downloading the CrossPack for AVR Development
[http://www.obdev.at/products/crosspack].
If you are using Linux, you will need to install avr-gcc, avr-libc, and AVRDUDE. Ubuntu users can get
the required software by running:
6. Getting started Page 49 of 62
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
sudo apt-get install gcc-avr avr-libc avrdude
After you have installed the prerequisites, open a command prompt and try running these commands
to make sure all the required utilities are available:
avr-gcc -v
avr-objcopy -V
make -v
avrdude
If any of those commands fail, make sure the desired executable is installed on your computer and
make sure that it is in a directory listed in your PATH environment variable.
Compiling an example program
Copy the following code to a file named “main.c”:
1
#define F_CPU 16000000
2
#include <avr/io.h>
3
#include <util/delay.h>
4
5
int main()
6
{
7
DDRC |= (1 << DDC7); // Make pin 13 be an output.
8
while(1)
9
10
11
12
13
14
15
{
PORTC |= (1 << PORTC7); // Turn the LED on.
_delay_ms(500);
PORTC &= ~(1 << PORTC7); // Turn the LED off.
_delay_ms(500);
}
}
In the same folder, create a file named “Makefile” with the following contents:
PORT=\\\\.\\USBSER000
MCU=atmega32u4
CFLAGS=-g -Wall -mcall-prologues -mmcu=$(MCU) -Os
LDFLAGS=-Wl,-gc-sections -Wl,-relax
CC=avr-gcc
TARGET=main
OBJECT_FILES=main.o
all: $(TARGET).hex
clean:
%.hex: %.obj
%.obj: $(OBJECT_FILES)
program: $(TARGET).hex
rm -f *.o *.hex *.obj *.hex
avr-objcopy -R .eeprom -O ihex $< $@
$(CC) $(CFLAGS) $(OBJECT_FILES) $(LDFLAGS) -o $@
avrdude -p $(MCU) -c avr109 -P $(PORT) -U flash:w:$(TARGET).hex
?
Make sure that the PORT variable in the Makefile is set to the name of the device’s virtual serial port.
6. Getting started Page 50 of 62
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
In Windows, \\\\.\\USBSER000 should work if the A-Star is the only USB device connected that is
using the usbser.sys driver, but you can change it to be the actual name of the COM port (e.g. COM13 ).
In a command prompt, navigate to the directory with the Makefile and main.c. If you run the command
make , the code should get compiled and produce a file named “main.hex”.
Programming
To program the A-Star device, you will need to get it into bootloader mode first. One way to do this is
to reset the AVR twice within 750 ms. Most of the boards in our 32U4 family have a reset button that
can be used to reset the board. On any of our 32U4 family of boards, a pushbutton can be connected
between the GND and RST pins to serve as a reset button, or you can use a wire. Once the device is
in bootloader mode, quickly run the command make program to program it. If you wait longer than 8
seconds, the A-Star bootloader will exit and the AVR will go back to running the user program.
6. Getting started Page 51 of 62
1. In the Arduino IDE, open the “Sketch” menu, select “Include Library”, then “Manage
Libraries…”.
2. Search for “AStar32U4”.
3. Click the AStar32U4 entry in the list.
4. Click “Install”.
1. Download the latest release archive from GitHub [https://github.com/pololu/a-star-32u4-arduino-
library] and decompress it.
2. Rename the folder “a-star-32u4-arduino-library-master” to “AStar32U4”.
3. Move the “AStar32U4” folder into the “libraries” directory inside your Arduino sketchbook
directory. You can view your sketchbook location by opening the “File” menu and selecting
“Preferences” in the Arduino IDE. If there is not already a “libraries” folder in that location,
you should make the folder yourself.
4. After installing the library, restart the Arduino IDE.
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
7. A-Star 32U4 Arduino library
The A-Star 32U4 can be programmed from the Arduino IDE as described in the preceding sections.
To help interface with all the on-board hardware on the A-Star 32U4, we provide the AStar32U4
library. The AStar32U4 library documentation [https://pololu.github.io/a-star-32u4-arduino-library/]
provides detailed information about the library, and the library comes with several example sketches.
If you are using version 1.6.2 or later of the Arduino software (IDE), you can use the Library Manager
to install this library:
If this does not work, you can manually install the library:
After you install the AStar32U4 library, you can learn more about it by trying the included example
sketches and by reading the AStar32U4 library documentation [https://pololu.github.io/a-
star-32u4-arduino-library/].
7. A-Star 32U4 Arduino library Page 52 of 62
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
8. The A-Star 32U4 USB interface
Our 32U4 family of boards are based on a single AVR ATmega32U4 microcontroller that runs the
user program and also handles the USB connection to the computer. The AVR has a full-speed USB
transceiver built into it and can be programmed to present almost any type of USB device interface to
the computer.
USB is an asymmetric system that consists of a single “host” connected to multiple “devices”. The host
is typically a personal computer. The ATmega32U4 can only act as a USB device, so an A-Star device
cannot be connected to other USB devices like mice and keyboards; it can only be connected to a
host such as your computer.
Programming an ATmega32U4 board using the Arduino IDE as described earlier will automatically
configure it as a composite device with a single virtual serial port. If you program the microcontroller
with an Arduino sketch that implements another USB device class, like HID or MIDI, you will see
additional child devices as well.
On a Windows computer, you can see the virtual serial port by going to your computer’s Device
Manager and expanding the “Ports (COM & LPT)” list. You should see a COM port labeled “Pololu A-
Star 32U4”. In parentheses after the name, you will see the name of the port (e.g. “COM3” or “COM4”).
Windows will assign a different COM port number to the device depending on what USB port you
plug it into and whether it is in bootloader mode or not. If you need to change the COM port number
assigned to the A-Star, you can do so using the Device Manager. Double-click on the COM port to
open its properties dialog, and click the “Advanced…” button in the “Port Settings” tab. From this dialog
you can change the COM port assigned to the device.
Windows 10 Device Manager showing the A-Star’s virtual COM port.
On a Windows computer, you can see the rest of the USB interface by going to the Device Manager,
selecting View > Devices by connection, and then expanding entries until you find the “Pololu A-Star
8. The A-Star 32U4 USB interface Page 53 of 62
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
32U4” COM port. Near it, you should see the parent composite device.
The Windows 10 Device Manager in “Devices by connection” mode, showing
that the A-Star is a composite device.
On a Linux computer, you can see details about the USB interface by running lsusb -v -d 1ffb: in
a Terminal. The virtual serial port can be found by running ls /dev/ttyACM* in a Terminal.
On a Mac OS X computer, the virtual serial port can be found by running ls /dev/tty.usbmodem* in a
Terminal.
You can send and receive bytes from the virtual serial port using any terminal program that supports
serial ports. Some examples are the Serial Monitor in Arduino IDE, the Pololu Serial Transmitter
Utility [https://www.pololu.com/docs/0J23], Br@y Terminal [http://sites.google.com/site/terminalbpp/], PuTTY
[http://www.chiark.greenend.org.uk/~sgtatham/putty/], TeraTerm [http://ttssh2.sourceforge.jp/], Kermit
[http://www.columbia.edu/kermit/ck80.html], and GNU Screen [http://www.gnu.org/software/screen/]. Many
computer programming environments also support sending and receiving bytes from a serial port.
8. The A-Star 32U4 USB interface Page 54 of 62
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
9. The A-Star 32U4 Bootloader
Our 32U4 family of boards come with a USB bootloader that can be used in conjunction with the
Arduino IDE or AVRDUDE to load new programs onto the device. This section documents some
technical details of the bootloader for advanced users who want to better understand how it works. If
you just want to get started using your device, it is fine to skip this section.
The A-Star 32U4 Bootloader is based on the Caterina bootloader [https://github.com/arduino/Arduino/tree/
master/hardware/arduino/avr/bootloaders/caterina], which is the bootloader used on the Arduino Leonardo
[https://www.pololu.com/product/2192], Arduino Micro [https://www.pololu.com/product/2188] and several other
ATmega32U4 boards. The bootloader is open source and its source code [https://github.com/pololu/
a-star/tree/master/bootloaders/caterina] is available on GitHub. The bootloader occupies the upper four
kilobytes of the ATmega32U4’s program memory, leaving 28 KB for the user program. The
bootloader’s USB interface consists of a single virtual serial port that accepts the programming
commands defined in AVR109 [http://www.atmel.com/images/doc1644.pdf]. The bootloader always runs first
immediately after the AVR is reset.
Startup logic
The main difference between the A-Star 32U4 Bootloader and Caterina is in the startup logic. This
is the part of the bootloader that runs immediately after the AVR is reset, and it decides whether to
run the user program or run the rest of the bootloader. The startup logic of the Caterina bootloader is
designed so that when the RST line goes low, the bootloader will run. This means that if you want to
restart your program using the RST line, it will take 8 seconds before the bootloader times out waiting
for an upload and the sketch starts.
The A-Star 32U4 Bootloader has different startup logic that allows you to use the RST line to reset the
board with a smaller delay. If the RST line goes low once, the user program will run after a 750 ms
delay. If the RST line goes low twice within 750 ms, then the bootloader will run. (This behavior is the
same as on boards like SparkFun’s Pro Micro.)
The start-up logic of the A-Star 32U4 Bootloader is shown in the flowchart below:
9. The A-Star 32U4 Bootloader Page 55 of 62
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
The startup logic for the A-Star 32U4 bootloader.
Brown-out detection
Unlike many other ATmega32U4 boards, our 32U4 family of boards have brown-out detection enabled.
The brown-out threshold is 4.3 V, and if the voltage on VCC goes below this then the AVR will reset.
9. The A-Star 32U4 Bootloader Page 56 of 62
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
The bootloader was designed so that the user program can detect brown-out resets. To do so, check
to see if the BORF bit in the MCUSR register is set, and then clear it later. Here is some example code
you could put in your setup function for detecting brown-out resets:
1
pinMode(13, OUTPUT);
2
if (MCUSR & (1 << BORF))
3
{
4
// A brownout reset occurred. Blink the LED
5
// quickly for 2 seconds.
6
for(uint8_t i = 0; i < 10; i++)
7
{
8
9
10
11
12
13
14
digitalWrite(13, HIGH);
delay(100);
digitalWrite(13, LOW);
delay(100);
}
}
MCUSR = 0;
?
9. The A-Star 32U4 Bootloader Page 57 of 62
• You accidentally loaded a malfunctioning program onto the device that is incapable of
responding to the special USB command. For example, your program might be stuck in an
infinite loop with interrupts disabled.
• You loaded a program which uses a non-standard type of USB interface or no USB interface.
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
10. Reviving an unresponsive A-Star
In order to load a new program onto your A-Star 32U4 device, you will need to get it into bootloader
mode and send programming commands to it over its virtual serial port using appropriate software. If
you are programming the device from the Arduino IDE, the sketch loaded onto the device will generally
support a special USB command for putting it in bootloader mode, and the Arduino IDE sends that
command automatically when you click the Upload button. However, you might find yourself in a
situation where the device is unresponsive and that method will not work. This can happen for two
reasons:
The following sections provide different procedures you can use to revive your device.
10.1. Reviving using the Arduino IDE
This section explains two special methods for programming an A-Star (or another of our 32U4 family
of boards) using the Arduino IDE in case your usual method of programming is not working. These
instructions were developed for the Arduino IDE versions 1.0.5-r2 and 1.6.0, and they might need to
be modified for future versions.
Reset button
If you have an A-Star 32U4 Micro, you should connect a momentary pushbutton
[https://www.pololu.com/product/1400] between the GND and RST pins to serve as a reset button. Other
boards in our 32U4 family have a reset button you can use. Alternatively, you can use a wire to
temporarily connect GND and RST together instead of using a reset button.
Resetting the board twice within 750 ms makes the board go into bootloader mode. The bootloader
will exit after 8 seconds and try to run the sketch again if it has not started receiving programming
commands. To revive the device, you need to make sure you start sending it programming commands
before the 8-second period is over.
In bootloader mode, the yellow LED (the one labeled LED 13) fades in and out. It is useful to look at
this LED so you can know what mode the microcontroller is in. Also, we recommend enabling verbose
output during upload using the Arduino IDE’s “Preferences” dialog. Looking at the LED and looking at
the verbose output during the following procedures will help you understand what is going on.
10. Reviving an unresponsive A-Star Page 58 of 62
1. Connect the device to your computer via USB.
2. In the “Tools” menu, open the “Board” sub-menu, and select “Pololu A-Star 32U4”.
3. In the “Tools” menu, open the “Port” sub-menu, and check to see if any ports are selected. If
the “Port” menu is grayed out or no ports in it are selected, that is good, and you can skip to
step 6.
4. Reset the board twice to get the board into bootloader mode. While the board is in bootloader
mode, quickly select the new serial port that corresponds to the bootloader in the “Port”
menu.
5. After 8 seconds, the bootloader will exit and attempt to run the sketch again. Wait for the
bootloader to exit. Verify that either the “Port” menu is grayed out or no ports in it are selected.
6. Click the Upload button. The Arduino IDE will compile your sketch and start uploading it.
7. As soon as the large status bar near the bottom of the IDE says “Uploading…”, reset the
board twice to get into bootloader mode.
1. Connect the device to your computer via USB.
2. In the “Tools” menu, open the “Board” sub-menu and check to see if the “Pololu A-Star 32U4
(bootloader port)” entry is visible. If this entry is visible, you can skip to step 6.
3. If you are using a 1.0.x version of the Arduino IDE, open the file [sketchbook
location]/hardware/pololu/boards.txt using a text editor. If you are using a 1.5.x version
of the Arduino IDE, open the file [sketchbook location]/hardware/pololu/avr/boards.txt
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
The uploading-before-bootloader method
The goal of the uploading-before-bootloader method is to select a non-existent serial port in the
Arduino IDE and then make sure the Arduino IDE enters the uploading phase before the
microcontroller goes into bootloader mode. This method has been tested on Arduino 1.0.5-r2 and
1.6.0. This method does not work on Arduino 1.5.6-r2 because that version of the IDE gives a fatal
error message if the selected serial port is not present at the beginning of the uploading phase (e.g.
“Board at COM7 is not available.”).
The Arduino IDE will stay in the uploading phase for 10 seconds, waiting for a new serial port to
appear. Once the serial port of the bootloader appears, the Arduino IDE will connect to it and send
programming commands.
The bootloader-before-uploading method
The goal of the bootloader-before-uploading method is to select the bootloader’s virtual serial port in
the Arduino IDE and then make sure the board is in bootloader mode at the time when the Arduino
IDE enters the uploading phase.
10. Reviving an unresponsive A-Star Page 59 of 62
using a text editor. You can see the sketchbook location in the Arduino IDE preferences
dialog. The file you are looking for is part of the A-Star add-on.
4. In the boards.txt file that you opened, find the lines at the bottom of the file that start with
#a-star32U4bp . Uncomment each of those lines by deleting the “#” character, and then save
the file.
5. Close the Arduino IDE and restart it.
6. In the “Tools” menu, open the “Board” sub-menu and select “Pololu A-Star 32U4 (bootloader
port)”. This entry is configured so that the Arduino IDE will send programming commands
directly to selected serial port, instead of trying to send a special USB command to the port
to get it into bootloader mode and then waiting for the new port to appear. By selecting this
entry, the timing of the programming process below becomes easier, especially on Windows.
7. Prepare the computer to show you a list of its virtual serial ports. If you are using Windows,
this means you should open the Device Manager. If you are on Linux or Mac OS X, this
means you should open a Terminal and type the command ls /dev/tty* but do not press
enter until the board is in bootloader mode in the next step.
8. Reset the board twice to get the board into bootloader mode. While it is in bootloader mode,
quickly look at the list of serial ports provided by your operating system in order to determine
what port the bootloader is assigned to.
9. Reset the board twice to get the board into bootloader mode again. While the board is in
bootloader mode, quickly select the serial port of the bootloader in the Arduino IDE. The port
can be selected in the “Port” sub-menu under “Tools”.
10. In the Arduino IDE, click the “Verify” button to compile your sketch. This could make the timing
easier during the next step.
11. Press the reset button twice to get the board into bootloader mode again. As soon as you
see the yellow LED fading in and out, press the Upload button.
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
The Arduino IDE will compile your sketch and then upload it to the selected serial port.
If the compilation of the sketch takes longer than 8 seconds, then this procedure will fail because the
bootloader will time out and start trying to run the malfunctioning sketch again. If that happens, try the
procedure again using a simpler sketch such as the Blink example that can be found under File >
Examples > 01.Basics > Blink.
After reviving your device, be sure to change the Board setting back to “Pololu A-Star 32U4” and select
the right Port.
10.2. Reviving using AVRDUDE
This section explains a special method for reviving an A-Star (or another of our 32U4 family of boards)
10. Reviving an unresponsive A-Star Page 60 of 62
1. Connect the device to your computer via USB.
2. Prepare the computer to show you a list of its virtual serial ports. If you are using Windows,
this means you should open the Device Manager. If you are on Linux or Mac OS X, this
means you should open a Terminal and type the command ls /dev/tty* but do not press
enter until the board is in bootloader mode in the next step.
3. Press the reset button twice within 750 ms to make the AVR go into bootloader mode. You
should see the yellow LED fading in and out when the AVR is in bootloader mode. While it is
in bootloader mode, quickly look at the list of serial ports provided by your operating system
in order to determine what port the bootloader is assigned to.
4. Type the following command in your terminal and replace COM4 with the name of the
bootloader’s serial port, but do not press enter yet. This command will erase the
malfunctioning program on the device but preserve the bootloader.
avrdude -c avr109 -p atmega32U4 -P COM4 -e
5. Press the reset button twice within 750 ms to make the AVR go into bootloader mode.
6. Quickly run the command you typed previously. The command needs to be run within
8 seconds of starting the bootloader, or else the bootloader will exit and try to run the
malfunctioning program again.
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
using the command-line utility AVRDUDE [http://www.nongnu.org/avrdude/] in case your usual method of
programming is not working. AVRDUDE stands for “AVR Downloader/UploaDEr”, and it is compatible
with the A-Star bootloader.
If you have an A-Star 32U4 Micro, you should connect a momentary pushbutton
[https://www.pololu.com/product/1400] between the GND and RST pins to serve as a reset button. Other
boards in our 32U4 family have a reset button you can use. Alternatively, you can use a wire to
temporarily connect GND and RST together instead of using a reset button.
By following the instructions above, the malfunctioning program on the device will be erased and the
device will stay in bootloader mode indefinitely. You can now load another program onto it using the
Arduino IDE or AVRDUDE.
10. Reviving an unresponsive A-Star Page 61 of 62
• The Arduino IDE has many examples [http://arduino.cc/en/Tutorial/HomePage] that can run on the
A-Stars.
• The Arduino website has a Language Reference [http://arduino.cc/en/Reference/HomePage], a
wiki called the The Arduino Playground [http://playground.arduino.cc/], and other resources.
• The A-Star boards are similar to the Arduino Leonardo [https://www.pololu.com/product/2192]
and Arduino Micro [https://www.pololu.com/product/2188], so you can search the Internet for
relevant projects that use one of those boards.
• Atmel’s ATmega32U4 documentation [https://www.microchip.com/wwwproducts/en/ATmega32u4]
has the ATmega32U4 datasheet and many related documents.
• AVR Libc Home Page [http://www.nongnu.org/avr-libc/]: this page documents the standard
library of functions that you can use with GNU C and C++ compilers for the AVR.
• A-Star 32U4 Arduino library [https://github.com/pololu/a-star-32u4-arduino-library]
• AStar32U4 library documentation [https://pololu.github.io/a-star-32u4-arduino-library/]
• LUFA – the Lightweight USB Framework for AVRs [http://www.fourwalledcubicle.com/
LUFA.php]
• WinAVR [http://winavr.sourceforge.net/]
• Atmel Studio 7 [https://www.microchip.com/avr-support/atmel-studio-7]
• AVRDUDE [http://www.nongnu.org/avrdude/]
• AVR Freaks [http://www.avrfreaks.net/]
• Texas Instruments TPS2113A power multiplexer datasheet [https://www.pololu.com/file/
0J771/tps2113a.pdf] (1MB pdf)
• Texas Instruments DRV8838 motor driver datasheet [https://www.pololu.com/file/0J806/
drv8838.pdf] (1MB pdf)
• Maxim MAX14870 motor driver datasheet [https://www.pololu.com/file/0J885/MAX14870.pdf]
(492k pdf)
Pololu A-Star 32U4 User’s Guide © 2001–2019 Pololu Corporation
11. Related Resources
To learn more about using the Pololu A-Star boards, see the following list of resources:
Datasheets for some of the components found on the A-Star 32U4 boards are available below (not all
components apply to every board):
Finally, we would like to hear your comments and questions on the A-Star section of the Pololu
Robotics Forum [http://forum.pololu.com/viewforum.php?f=10]!
11. Related Resources Page 62 of 62
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